The skin is our largest organ and forms a fascinating and unique interface between us and the outside world. The stratum corneum, the outermost keratinized layer of thick-walled epidermal cells, is the most important, as this serves as the barrier to both the ingress of chemicals and other agents, microorganisms and dangerous substances, and the egress of water. However, the skin is not a total barrier and transdermal absorption can play a considerable role in the internal exposure of persons exposed to hazardous substances. Some chemicals are more toxic topically than orally, at least in animals. Furthermore, many compounds are absorbed to a greater degree from the skin than orally.
Dermal exposure to chemicals occurs in a wide variety of occupations, spanning agriculture, manufacturing, and industrial fields. Pesticides, solvents, and polycyclic aromatic hydrocarbons are some of the main chemical groups that have been recognized as posing health problems by dermal absorption. Due to pesticides, low volatility, and persistence, the amount of material inhaled is likely to be low unless a particularly vigorous application results in significant aerosol formation. Workers in market gardens and greenhouses can experience high dermal exposures during application or harvest where handling of vegetation coated with pesticide residues takes place. Despite widespread use of solvents, they are able to irritate and permeate the skin and affect a number of target organs within the body, including the kidneys, liver, and nervous system. As solvents tend to be volatile, their toxicity may principally result from inhalation of vapor. However, the highly lipophilic nature of most solvents can also result in dermal uptake when deposited on the skin. On the other hand, chronic exposure to solvents, which is inevitable in many occupations, may lead to an impairment of the skin barrier, so toxic substances are allowed to reach the reservoir of the stratum corneum, or even deeper layers of the skin. Skin represents a significant route of entry for many chemical warfare agents, including sulphur mustard (a skin damaging agent) and VX (an anticholinesterase or “nerve” agent) which represent a potential hazard to both public service and civilian populations and have allegedly been used by military and terrorist organizations. Many other materials may also be absorbed through the skin in significant amounts. These include mercury, isocyanates, polychlorinated biphenyls, acrylates, and pharmaceutical products such as steroids and nicotine.
On the other hand, percutaneous absorption can be increased in various ways, such as by the application of skin product on damaged skin, heat, and other mechanisms that all can worsen the problem. In this view, personal protective equipments, including specific suits, face masks, gloves and overboots, provide an efficient protection against the liquid and vapor forms of most toxic chemicals. However, due to their relative tightness, protective equipments may induce physical and heat stress. Moreover, many gloves do not resist the penetration of low molecular weight chemicals. Some allergens are soluble in rubber gloves and can penetrate the glove and induce severe dermatitis. Furthermore, the glove membrane can be structurally modified by a solvent; this may lead to changes in permeation behavior. For all these reasons, the “topical skin protectant” strategy has been adopted. Theoretically, skin barrier creams retard or even prevent the penetration into the skin. These creams can be seen as reinforcing the natural barrier function of the skin and they are developed to complement or replace protective equipments. However, different studies have revealed that often barrier creams do not fulfill their protecting behavior completely. It has been shown that barrier creams can be considered to give poor skin protection against the organic solvents investigated. Even some studies demonstrated penetration enhancement of the model penetrants through skin treated with barrier creams compared to untreated skin. It is for these reasons that there have been attempts to improve their efficacy. Special effort has been put on developing active substances that reduce percutaneous absorption of hazardous materials. It has been shown that beta-cyclodextrins may be usefully incorporated into a barrier formulation to reduce percutaneous absorption of toxic materials on occupational exposure. Permeation retardation may be due to complexation. In another study, a barrier cream coded as HP01 contains reactive protectants that chemically react with sulphur mustard. It was also shown that β-cyclodextrin and polyethylene glycol 1540 decreased the permeation of nitroglycerin significantly by about 2-4 times. The retardation effect is possibly due to hydrogen bonding between the model penetrant and the interacting polymers.